Radius Drawing Tool

Nobody has claimed this yet.

We've implemented a custom draw mode for drawing a radius, and are wondering if there is interest in a PR.

You can see it in action in [NYC Population Factfinder](https://popfactfinder.planning.nyc.gov/), and read a blog post about it [here](https://medium.com/nycplanninglabs/building-a-custom-draw-mode-for-mapbox-gl-draw-1dab71d143ee).

Some things to consider:
- We use this as a selection tool, so it never actually adds anything to the drawn features the way the existing tools do.
- Should it add a feature? If so, what feature would it add, and how should it be styled? (See the blog post, the radius is defined by a center point and a distance, not really a circle, but the feature we add to the drawn features could be either a `Point` visualized using a mapboxGL circle layer, or a circle-like polygon.
- The circle-like polygon may be a better choice because it will remain an accurate representation of the radius when the user pitches the map.
- Our implementation includes measurements as a symbol layer as the user draws. Any interest in including this in mapbox-gl-draw?

No repository file, test, or entry point is named. Start by reviewing the existing draw modes and the linked custom implementation and blog post, then clarify whether radius selection should create a feature and how measurements should be represented and styled. Done means the project agrees on those design decisions and defines an implementable scope.

Written by the indexing model from the issue text.